1. Field of the Invention
The present invention relates to a clock generation circuit for frequency-modulating an output clock, and more particularly, to a Spread Spectrum type clock generation circuit whose electromagnetic interference (hereinafter referred to as EMI) for peripheral equipment is reduced.
2. Related Background Art
Conventionally, as EMI measures, a clock generator having a clock modulation circuit may be employed in many cases in order to reduce the number of components such as ferrite beads and a passive filter in a case where the frequency of a system operation clock exceeds 10 MHz. However, the higher frequency modulation efficiency is in the clock generator, the more effective the use of the clock generator is as the EMI measures.
In a case where frequency modulation is performed with respect to a general clock generation circuit, modulation efficiency in the frequency modulation indicates how much the peak value is reduced when the respective spectra of an output clock in a case where no modulation is performed and a case where modulation is performed are compared with each other. That is, the smaller the peak value becomes, the better the modulation efficiency is.
The modulation efficiency varies depending on the period and the amplitude of a modulation waveform. It is said that the modulation efficiency is best when the period of the modulation waveform is generally 20 kHz to 50 kHz. The larger the amplitude of the modulation waveform is, the higher the modulation efficiency becomes. When the amplitude is increased, however, the frequency of the output clock greatly swings, so that jitter is increased. When the jitter is too high, an IC using the output clock may be erroneously operated. Therefore, a method of increasing the amplitude of the modulation waveform to improve the modulation efficiency has a limit. Thus, the frequency modulation efficiency has been conventionally improved by changing the modulation waveform.
A first example of modulating methods conventionally known is a method of modulating a control signal to be fed to a voltage controlled oscillator (hereinafter referred to as VCO) in a phase-locked loop (hereinafter referred to as PLL), that is, an output of a charge pump by an analog circuit, to generate a modulated clock by intentionally changing the frequency of an output clock (see Japanese Unexamined Patent Publication No. 09-098152, U.S. Pat. No. 5,631,920).
It is generally said that as the inclination of a modulation wave form increases toward its peak portion, while decreasing toward its intermediate point when the amplitude and the period of the modulation waveform are respectively the same, the modulation efficiency thereof increases. In the above-mentioned conventional example, therefore, a modulation waveform generation apparatus capable of generating not a simple triangular waveform or sine waveform but a complicated waveform commonly called “hershey-kiss waveform” shown in FIG. 1 is employed in order to satisfy the above-mentioned inclination.
A second example of modulating methods conventionally known is a method of modulating the frequency division number of a frequency divider receiving an output of a VCO. In the second conventional example, it is possible to construct all constituent elements from one for generating a modulation waveform to one for adding a modulation component to the frequency division number of the frequency divider by a digital circuit and to generate a modulation component signal having a small number of steps of quantization by employing ΔΣ modulation to minutely change the modulation amplitude thereof (see Japanese Unexamined Patent Publication No. 2001-117662, U.S. Pat. No. 6,559,698).
In the above-mentioned first conventional example, a modulation signal is generated using the analog circuit, or using a digital-to-analog converter. Therefore, the modulation waveform is frequently distorted. Degradation of the frequency modulation efficiency due to the distortion second conventional example, the modulation waveform is generated by the digital circuit. From the viewpoint of eliminating the need for a complicated circuit, the modulation waveform is mainly a triangular waveform shown in FIG. 2. Since the slope of the triangular waveform is constant, however, the triangular waveform is inferior in frequency modulation efficiency.